In lean burn engine control systems, lean NOx traps may be used to stored NOx produced during lean exhaust air-fuel operating conditions and thereby reduce emitted NOx from the tailpipe. Then, when the NOx trap is sufficiently full, for example, the exhaust air-fuel ratio may be switched to a stoichiometric or rich air-fuel ratio to purge stored NOx, as well as any other stored oxidants, such as oxygen.
The air-fuel ratio of the exhaust gas during the purging operation (referred to as the purge air-fuel ratio) may be selected based on various parameters to attempt to lower the amount of unreacted NOx released from the tailpipe. In one example, the purge air-fuel ratio may be selected based on temperature and/or a total amount of NOx or oxidants stored in the trap. Such an approach is based on a lumped parameter model, which assumes that the NOx and any other oxidants, such as oxygen, are substantially evenly distributed along various axes of the trap, such as the axial length of the trap.
The inventors herein have recognized that this may lead to several disadvantages. Specifically, some oxidants, such as NOx, may be more unevenly stored along various axes of the trap than other oxidants, such as oxygen. For example, significantly more NOx may be stored in trap's upstream portion than in the trap's downstream portions relative to oxygen. Further, the profile of NOx storage along the axial length of the trap may also vary under different conditions. These effects may also be exacerbated if an emission control device is used that has uneven precious metal loading.
At least some of the above issues may be addressed by a method for controlling a lean burn engine coupled to an emission control device that stores oxidants during lean operation, and reacts to the stored oxidants during stoichiometric or rich operation, the method comprising:
estimating amounts of NOx stored in the device along a plurality of axial positions of the device; and
adjusting an operating parameter based on said estimate.
In this way, it is possible to provide improved emission control by taking into account where NOx is stored, as well as, how much NOx is stored, for example. This may enable improved NOx purging operation since the amount or rate of reductants supplied over time may be tailored to better match the distribution of NOx along the axial position of the emission control device, in one example. Various other approaches may also be used, as described in more detail herein.